Concretes of standard DIN 1045, Eurocode 2, ACI 318-89, are distinguished by their bulk density: lightweight concrete, normal weight concrete and heavy weight concrete. These concretes are substantially manufactured from binders--in general cement of standard DIN 1164 or comparable standards--aggregate, which meets the requirements of DIN 4226 (part 1-3) and water. Concrete admixtures and additives are usually additionally used. The first mentioned substances change the characteristics of the concrete by chemical and/or physical action, e.g. solidification, workability or setting. In contrast, additives are finely-distributed additional substances which influence specific concrete properties and in contrast to the concrete admixtures have to be taken into consideration as parts by volume when calculating the mixing ratios, e.g. latent hydraulic substances or pigments, which can also be of organic origin.
Prefabricated members and buildings made of concrete--reinforced concrete members or prestressed concrete members--must meet a number of requirements in respect of bearing capacity and stability under load. The pertinant standards (inter alia DIN 1045 and DIN 4227, Eurocode 2, etc) and the building regulations of the land have to be taken into account in design and manufacturing the same.
Prefabricated concrete members and concrete buildings must also meet a number of requirements in respect of fire safety. In this connection, the building regulations of the land and, in particular, the standards DIN 4102 (Eurocode 2, etc) are decisive.
The stability of the concrete is impaired under fire attack and the prefabricated parts exhibit failures after being exposed to fire for a specific period of time. According to the concrete-fire protection handbook by K. Kordina and C. Meyer-Ottens, Betonverlag GmbH, Dusseldorf 1981, pages 152 to 167, in particular the following kinds of failures are observed in reinforced concrete members under fire: failure of the tension zone, failure due to thrust or torsional breakage, failure of the compression zone, failure by exceeding the admissible raise of temperature at the non fire-exposed surface and failure due to spalling.
Destructive spalling in prefabricated concrete members of normal strength could be counteracted by an appropriate selection of the dimensions, the cross-sectional shapes, the mechanical stress distribution and the arrangement of the reinforcement, in connection with their long-term drying in the building.
Practical experience and material tests show that until now, explosion spalling of prefabricated high-strength concrete members have always occured under fire exposure. The term high-strength concretes includes those, which, with respect to their strength, are superior to the highest strength class B 55, embraced by the standards DIN 1045, Eurocode 2 ACI 318-89 etc, e.g. a B 85. In order to obtain the high strength of the cement stone, high-strength concretes are made with very low water/cement ratios generally below 0.40. Concretes are impermeable to liquid water and their diffusion of water vapor takes place very slowly, such that the concrete--even after hundreds of year storage at ambient conditions--usually contains more than 3 weight-% water. This means that prefabricated members made of a high-strength concrete can never dry out under normal ambient conditions (.ltoreq.2% by weight).
Due to the prevailing high moisture content and the high diffusion resistance vis a vis water vapor, very high pressures necessarily result inside the high-strength prefabricated concrete members under fire, which finally lead to explosive spalling, in particular when the concrete is simultaneously subjected to high mechanical stress.
Spalling under fire has generally been observed in prefabricated members whose inherent moisture and impermeability exceed certain limits, for example .gtoreq.2% in normal strength concrete. Spalling also occurs in prefabricated parts of shotcrete (according to the standard DIN 18551) or in centrifugal concrete, light-weight concrete with closed structure and shot mortar.
For this reason, very narrow limits are set to the use of these building materials, in particular of high-strength concrete, or very expensive technical measures, such as an outer network reinforcement for preventing the falling off of the detached or spalled concrete core or expensive insulations against the fast penetration of heat in the case of fire are necessary. Also the addition of steel fibers to increase the tensile strength of the concrete did not lead to the desired success.